Stationary induction electrical apparatus



May 17, 1960 cs. CAMILLI 2,937,349

STATIONARY INDUCTION ELECTRICAL APPARATUS Filed Oct. 12, 1955 2Sheets-Sheet 1 May 17, 1960 e. CAMlLLl 2,937,349 STATIONARY INDUCTIONELECTRICAL APPARATUS Filed Oct. 12, 1955 2 Sheets-Sheet z United StatesPatent STATIONARY INDUCTION ELECTRICAL APPARATUS Guglielmo Camilli,Pittsfield, Mass., assignor to General Electric Company, a corporationof New York Application October 12, 1955, Serial No. 540,135

7 2 Claims. (Cl. 336-58) This invention relates to stationary inductionelectrical apparatus, and more in particular to means for improving thedielectric strength of stationary electrical induction apparatusemploying a halogenated fluid as a dielectric medium.

In the past it has been common to employ halogenated liquid compounds asdielectric medium in electrical apparatus, such as power transformersand the like, in order to provide improved insulation characteristics. Atypical compound of this type is askarel, although recently gaseouscompounds, such as sulphur-hexafluoride (SP octafluoropropane (CgFa),and decafluorobutane (C F have shown promising possibilities for suchuse.

In general it may be said that the dielectric strength of an insulatingmedium disposed between a pair of electrodes having different potentialsincreases proportionally with the increase of spacing between theelectrodes. Although in the case of non-halogenated dielectric fluidssome diflerence exists between the proportionality in uniform andnon-uniform electric fields, there is still an appreciable increase inlow frequency and impulse dielectric strength of the non-halogenatedfluid with increases in electrode spacing in non-uniform fields.

This generalization has been found to hold true for halogenated fluiddielectrics in the case of uniform electrical fields, and also in thecase of non-uniform electrical fields having a'low frequency (e.g. 60cycles per second). It has now been found, however, that the impulsestrength of a halogenated fluid in a non-uniform electrical field doesnot increase appreciably with increases in electrode spacing beyond acertain distance. In the case of the previously mentioned halogenatedfluids, this distance is about two inches.

Thus although better dielectric-strength characteristics are obtainedinelectrical apparatus generally by the use of halogenated fluiddielectric compounds (as compared with non-halogenated compounds), theimpulse strengths of the halogenated compounds in non-uniform fieldscannot be appreciably increased by increasing the electrode spacing. Inan electrical structure such as a transformer having a magnetic core anda plurality of windings, there are generally many places where theelectrical fields are non-uniform, and as a result a serious limitationis placed uponthe use of halogenated fluids in such transformers forhigh voltage applications.

It is therefore an object of this invention to provide means forimproving the dielectric strength characteristics of stationaryelectrical induction apparatus immersed in a halogenated -dielectricfluid medium.

Another object is to provide means for obtaining sub stantially uniformelectrical fields in stationary electrical induction apparatus immersedin a halogenated dielectric fluid medium.

Still another object of this invention is to provide 'means instationary electrical induction apparatus immersed in -a halogenateddielectric fluid for simulating sphere gaps in regions of highelectrical stress in order to increase the impulse dielectric strengthof the dielectric fluid.

A further object is to provide shield means in a stationary electricalinduction apparatus immersed in a di electric fluid, the shield meansbeing disposed in regions of non-uniform electrical fields in order tosimulate sphere gaps and thereby increase the impulse dielectricstrength of the dielectric fluid.

Briefly stated, in accordance with one embodiment of my invention, Iprovide a stationary electrical induction apparatus, such as thetransformer or reactor, comprised of a magnetic core having at least oneleg member adapted to receive electrical windings, the leg member beingjoined at its ends to yoke members. A high voltage winding is providedsurrounding the leg member, and electrically insulated therefrom. Inorder to reduce the non-uniformity of the electrical fields between thehigh voltage winding and the yoke, I provide a shield extending in aplane perpendicular to the plane of the leg, and axially spaced from theend of the high voltage winding. The shield preferably extends in theplane at least as far as the radial outermost extremity of the highvoltage winding, and the outermost edges of the shield is provided witha substantially uniform curvature away from the high voltage winding.The shield is electrically connected to the core. The entire apparatusis immersed in a halogenated dielectric fluid medium such as askarel orsulphur-hexafiuoride.

I also prefer to provide an annular shield member at the ends of thehigh voltage winding. The annular shield member has semicircular crosssection and the flat side of the annular shield member is disposedfacing the high voltage winding. The annular shield member iselectrically connected to an end turn of the high voltage winding, andmay surround an end turn or turns of the high voltage winding.

In the modification of my invention wherein a low voltage winding isdisposed between the winding leg and the high voltage winding, I preferto employ an annular shield member at the ends of the low voltagewinding. The low voltage annular shield member preferably extends fromthe radially outermost extremities of the low voltage winding, and isprovided for at least a portion of its axial dimension with asubstantially uniform curvature extending radially inward.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing,

Fig. l is a partially cross sectional view of a transformer embodyingthe shield means of my invention,

Fig. 2 is a partially cross sectional view along the lines 22 of thetransformer of Fig. i,

Fig. 3 is an enlarged partially cross sectional view along the lines 3-3of the transformer of Fig. 1,

Fig. 4 is an impulse breakdown voltage versus gap spacing for askarel,and

Fig. 5 is an impulse breakdown voltage versus gap spacing forsulphur-hexafluoride.

Referring now to the drawings, and more in particular to Figs. 1 and 2,therein is illustrated a transformer 10 enclosed in a halogenateddielectric fluid filled tank 11. The transformer 10 has a magnetic core12 comprised of upper and lower yoke members 13 and 14 respectivelyseparated by three parallel winding leg members (one of which is shownby the dashed lines in Fig.

=3 2). n the winding leg members are disposed three windings 15, 16 and17.

The windings 15, 16 and 17 are surrounded by coaxial cylinders 18, 19and 20 respectively of insulating material, the cylinders having axiallyextending openings 21 on one side of the windings and ducts 22 on thediametrically opposite sides of the windings. An external cooling andcirculating system (not shown) of conventional type is provided to forcethe dielectric fluid to circulate into the bottom of the tank 11 throughthe inlet opening 23, upwardly through the ducts 22, axially andradially through the windings, through the cylinder openings 21 into thetank 11, and thence through the outlet opening 24 in the upper portionof the tank to the circulation system. A baflle 25 is provided toprevent passage of the fluid upwardly through the tank 11 bypassing thewindings, and other baflle means may be provided in order to furtherdirect the flow of fluid. For example, axial spacers in the windings mayprovide a zig-zag path for the fluid across the windings in the mannerdisclosed in U.S. Letters Patent No. 2,632,041 which issued on anapplication of W. l. Bilodeau and is assigned to the assignee of thepresent invention.

The ends of the windings are spaced from the core yokes 13 and 14 bymeans of annular insulating members 26 separated by insulating blocks27. Shield members 23 and 29 are disposed in proximity to yoke members13 and 14 respectively, and are also spaced from the respective ends ofthe windings 15, 16 and 17.

Referring now to Fig. 3, therein is illustrated an enlarged partiallycross sectional view of a portion of the transformer of Fig. 1 takenalong the lines 33. In Fig. 3 the windings and shield members are shownin section, while the core members are shown unsectioned for clarity. Acore leg 30 is therein shown extending coaxially through the windings15. An insulating winding cylinder 31 surrounds the core leg 30, and alow voltage winding 32 is wound on the cylinder 31. The low voltagewinding is preferably a layer winding as illustrated in Fig. 3, and maybe comprised of several radially spaced apart portions, although othertypes of windings may also be employed. Axially extending spacers 33separate the radially outermost surface or extremity of the low voltagewinding from a coaxial insulating cylinder 34. Another insulatingcylinder 35 is radially spaced from the cylinder 34 by means of axiallyextending spacers 36, and a high voltage insulating winding cylinder 37is radially spaced from the cylinder 35 by means of axially extendingspacers 38. The high voltage winding 39 is preferably comprised of aplurality of axially separated disk-shaped coils and is coaxiallymounted on the high voltage winding cylinder 37, each of the coils beingcomprised of a plurality of turns of insulated conductor strands.

An annular shield is provided on the end of the high voltage winding,the shield being generally semicircular in cross section with its flatside toward the high voltage Winding. As shown in Fig. 3 the shield 45may surround the endmost coil 46 of the high voltage winding, and may becomprised of a metallizecl tape wrapped around the endmost coil 46 and aplurality of annular members 47 of insulating material of decreasingradial dimension away from the high voltage winding and stacked on theendmost coil 46 of the high voltage winding. The shield 45 is connectedto one turn of the endmost coil 46, such as the radially outermost turnconnected to lead 43 as shown in Fig. 3. The shield 45 is surrounded byan insulating material 49, which may consist of an insulating tapewrapped therearound.

The low voltage windings 32 extend axially beyond the high voltagewinding, and insulating cylinders 59 are provided abutting the endsthereof. The radially outermost insulating cylinder 53 is provided withan annular conducting shield 51 extending upwardly from the outermostextremity of the low voltage winding for a short .1. distance and thenceradially inwardly with a substantially uniform curvature. The shield 51may consist of a metallized surface on the outermost insulating cylinder50.

An annular insulating plate 52 with a cylindrical extension on itsradially inward edge extends between the respective ends of cylinder 35and cylinder 18, and is axially spaced from the shield 45 on the end ofhigh voltage winding 33 by insulating blocks 53. Annular insulatingplate 54 is axially spaced from insulating plate 52 by means ofinsulating blocks 55, and has a cylindrical extension on its radiallyinward edge joining the insulating cylinder 34.

Another annular insulating plate 56 is spaced from the 2 insulatingplate 54 by means of insulating blocks 57,

and extends inwardly to join the outermost of the insulating cylinders50. Still another annular insulating plate 58 is provided extending atleast from the outermost extremity of the low voltage winding to theoutermost extremity of the high voltage winding, and is axially spacedfrom the plate 56 and cylinders 50 by radially extending insulatingspacers 59 and 60 respectively.

The shield 28 has a flat central portion lying in a plane perpendicularto the axis of the core leg 30, and a curved outer portion 66 extendingwith a substantially uniform curvature away from the high voltagewinding. The flat central portion of the shield 28 extends at leastbetween the outermost extremity of the low voltage winding to theoutermost extremity of the high voltage winding, and has a centralaperture through which the magnetic core extends. The flat centralportion 65 of the shield 28 is held against the insulating plate 58 by aclamp means which may be comprised of blocks 70 having threadedextensions 71 bolted into angle members 72 of the core bracing assemblyof the magnetic core 13. The clamping assembly provides axial forces tohold the low and high voltage windings in place.

In order to further simulate a rounded surface at the ends of the highvoltage winding, and thus reduce the danger of sparkover between theends of the high and low voltage windings, the several coils of the highvoltage winding adjacent the ends of the winding have increasing radialspacing from the low voltage winding toward the end of the high voltagewinding. This may be accomplished, as illustrated in Fig. 3, byproviding annular insulated spacing members 73 of increasing radialthickness toward the end of the high voltage winding between thecylinder 37 and the inside edges of the coil 40 adjacent the end of thehigh voltage winding. The contour of the outside of the high voltagewinding is not as important in regard to danger of sparkover, and may bestraight as shown in Fig. 3.

Referring still to Fig. 3, there are several circulation paths for thedielectric fluid through the windings. A portion of the fluid is forcedupwardly in the duct 22, and thence horizontally across the faces of thehigh voltage coils and out through the opening 21. Other portions of thefluid are forced upwardly between the insulating cylinders 35 and 37,between the insulating cylinders 34 and 35, between the low voltagewinding and the cylinder 34, and between the radially spaced sections ofthe low voltage winding. The axially extending spacers 33, 36 and 38preferably do not extend beyond the end of the high voltage winding, inorder to reduce the stress on the dielectric fluid resulting in adifference in permittivity between the fluid and the solid insulation.This is especially important when gaseous insulation is employed.

Referring now to Fig. 4, the curves 80, 81 and 82 are impulse breakdownvoltage versus gap spacing characteristics for askarel, l0-C oil, andair respectively using onehalf inch square brass electrodes. The curves83 and 84 are similar curves for askarel and oil respectively using 12.5centimeter spherical electrodes. The voltage impulses were 1.5microsecond in duration and were separated from each other by about 40microseconds. From these curves it may be seen that askarel has betterimpulse characteristics than oil with spherical electrodes, or in otherwords in a substantially uniform field, while oil has better impulsecharacteristics than askarel in the nonuniform field provided by thesquare electrodes. From this curve it may also be seen that the impulsestrength of askarel does not increase appreciably for gap spacings aboveabout two inches. Extension of the curves 80 and 82 indicates that aspacing of about 13 inches or greater,

the impulse strength of air is better than that of askarel in anon-uniform electrical field. In Fig. curves 85 and 86 illustrate theimpulsebreakdown voltage versus gap spacing characteristics forspherical and square electrodes respectively in sulphur-hexafluoride.This curve illustrates that sulphur-hexafluoride, like askarel, providessubstantially no increase in impulse strength above about two-inch gapspacings in a non-uniform field, while the impulse strength is a linearfunction of the gap spacing in a uniform field.

In view of these characteristics of halogenated dielectric fluids, thepresent invention provides means for reducing the non-uniformity of theelectrical field in high stress regions of stationary electricalinduction apparatus in order to improve the impulse strength of theapparatus. Thus, in the transformer illustrated in Fig. 3, smoothcontours have been provided on the core and windings by providing adish-shaped shield adjacent the ends of the winding and electricallyconnected to the core, a semicircular cross section annular shield onthe ends of the high voltage winding, and a rounded annular shield onthe ends of the low voltage winding. Further improvement results inincreasing the radial spacing between the respective ends of the highand low voltage windings.

Although this invention has been disclosed with particular reference toa three phase transformer, it is obvious that the principles involvedmay be employed without departing from the spirit and scope of theinvention in other stationary electrical induction apparatus such assingle phase transformers.

It will be understood, of course, that, while the form of the inventionherein shown and described constitutes a preferred embodiment of theinvention, it is not intended herein to illustrate all of the possibleequivalent forms or ramifications thereof. It will also be understoodthat the words used are words of description rather than of limitation,and that various changes may be made without departing from the spiritor scope of the invention herein disclosed, and it is aimed in theappended claims to cover all such changes as fall within the true spiritand scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A stationary electrical induction apparatus comprising an enclosedtank, a halogenated dielectric fluid within said tank, a magnetic coreimmersed in said fluid and having a plurality of parallel leg members atleast one of which is adapted to receive electrical windings, said legmembers being joined at their respective ends by a pair of yoke members,a low voltage winding surrounding said one leg member and insulatedtherefrom, a high voltage winding surrounding said low voltage windingand radially spaced therefrom, said high voltage winding being comprisedof a plurality of axially spaced apart diskshaped coils each having aplurality of individual turns, first annular shield means having asubstantially semicircular cross section mounted coaxially on one end ofsaid high voltage winding and having its flat side toward said highvoltage winding, said first annular shield means being electricallyconnected to a turn of the endmost coil on said one end of said highvoltage winding, the several coils of said high voltage winding adjacentsaid one end having increasing radial spacing from said low voltagewinding toward said one end of said high voltage winding, one end ofsaid low voltage winding extending axially beyond said one end of saidhigh voltage winding, a

second annular shield means mounted coaxially on said one end of saidlow voltage winding and connected to a turn on said one end of said lowvoltage winding, said second annular shield means having a cylindricalportion having one end adjacent said one end of said low voltage windingand extending coaxially from said one end of said low voltage windingand a curved portion having a substantially uniform curvature extendingradially inward from the other end of said cylindrical portion, and adish-shaped shield member having a'flat central portion and a curvedoutermost portion, said shield member being axially spaced from saidannular shield means with said fiat portion lying in a planeperpendicular to the axis of said leg member, said shield having anaperture through which said core extends, said shield member beingelectrically connected to said core, said flat portion extending atleast from the radially outermost extremity of said low voltage windingto the radially outermost extremity of than the curvature of said firstannular shield means, said annular shield means and dish-shaped shieldand the increasing radii of said coils toward said one end of said highvoltage winding being proportioned to provide a substantially uniformelectrostatic field surrounding said high voltage winding.

2. A stationary electrical induction apparatus compris ing an enclosedtank, a halogenated fluid within said tank, a magnetic core immersed insaid fluid and having a plurality of parallel winding leg members joinedat their respective ends by a pair of yoke members, a low voltagewinding surrounding each of said leg members, a high voltage windingsurrounding each of said low voltage windings and being radially spacedfrom their respective lowvoltage winding, said high voltage windingsbeing each comprised of a plurality of axially spaced apart diskshapedcoils each having a plurality of individual turns, first annular shieldmeans mounted coaxially adjacent each end of each of said high voltagewindings, second annular shield means mounted coaxially adjacent eachend of said low voltage windings, said first annular shield means eachhaving a substantially semicircular cross section and being electricallyconnected to a turn of the respective end coil of said high voltagewindings, said first annular shield means having their flat sides towardthe ends of the respective high voltage windings, said second annularshield means each being electrically connected to a turn on the end ofthe respective low voltage winding and having a cylindrical portionhaving one end adjacent one end of said low voltage winding andextending from the radially outermost extremity of said respective lowvoltage winding and a curved portion extending from the other end ofsaid cylindrical portion and having a substantially uniform curvatureextending radially inwardly from said cylindrical portion and away fromsaid low voltage winding, the several coils of said high voltagewindings toward the ends of said high voltage windings having increasingradial spacing from their respective low voltage windings toward theends of said high voltage windings, and a dish-shaped shield memberhaving a flat central portion and a curved outermost portion disposed ateach end of said windings and axially spaced from said annular shieldmeans, said flat portions lying in planes perpendicular to the axes ofsaid leg members and extending at least from the radially outermostextremities of said low voltage windings to the radially outermostextremities of said high voltage windings, apertures through said shieldmembers through which said core extends, said shield members beingelectrically connected to said core, said outermost portions havingsubstantially uniform curvature away from said windings and less thanthe curvature of said first annular shield means, said shield means andshield member and the inner radii of said high voltage windings beingproportioned to provide a substantially meme 7 uniform electrostaticfield surrounding each end of each 2,153,090 of said high voltagewindings. 2,632,041

References Cited in the file of this patent UNITED STATES PATENTS 5243,698 1,368,811 Kurda Feb. 15 1921 344,181 1,549,891 Petersen Aug. 18,1925 405,441

8 Libbe 1 Apr. 4, 1939 Bilodeau Mar. 17, 1953 FOREIGN PATENTSSwitzerland Jan. 16 1947 Great Britain Mar. 5, 1931 Germany Nov. 5, 1924

